1. Field of the Invention
The present invention relates to a steam turbine used for thermal power generation and the like, and more particularly to a turbine blade assembly which is provided with axial inserted blade root type turbine blades which have blade root portions implanted by inserting in an axial direction of a turbine rotor and to a steam turbine provided with it.
2. Description of the Related Art
For the turbine blade, for example, strength design against high centrifugal forces, and vibration design to suppress the occurrence of resonant oscillation of the turbine blade due to an external force such as steam are important in addition to fluid design as a steam passage component element.
For the strength design, a connected portion structure with a turbine blade to be attached to an outer peripheral tip end of a rotor blade wheel of the turbine rotor is most important, and generally connected via a blade root portion which is formed by concavo-convex interlocking. The blade root fitting structure is divided roughly into two systems of a saddle-shaped blade root type and an axial inserted blade root type and being used extensively as disclosed in, for example, JP-A 9-177502 (KOKAI) and JP-A 2006-283681 (KOKAI).
FIG. 14 is a perspective view showing a rotor blade wheel 300 and turbine blades 310 to illustrate a blade root fitting structure according to a conventional saddle-shaped blade root type. FIG. 15 is a perspective view showing a rotor blade wheel 350 and turbine blades 360 to illustrate a blade root fitting structure according to a conventional axial inserted blade root type. FIG. 16 is a diagram showing a cross section of the rotor blade wheel 300 and the turbine blade 310 according to the conventional saddle-shaped blade root type viewed in a turbine rotor axial direction. FIG. 17 is a diagram showing a cross section of the rotor blade wheel 350 and the turbine blade 360 according to the conventional axial inserted blade root type viewed in a turbine rotor axial direction. FIG. 18 is a plan view of the rotor blade wheel 350 and the turbine blades 360 with stop blades fixed with screws viewed in the turbine rotor axial direction according to the conventional axial inserted blade root type. FIG. 19 is a plan view showing a structure of shroud portions 390 according to a conventional snubber type blade. FIG. 20 is a diagram showing a cross section of a blade root portion 370a of a standard rotor blade 370 and a rotor blade wheel 395 having a structure to control a radial directional position of the snubber type blade. FIG. 21 is a diagram showing a cross section of a blade root portion 380a of an offset rotor blade 380 and the rotor blade wheel 395 having a structure to control a radial directional position of the snubber type blade.
As shown in FIG. 14, the saddle-shaped blade root type is a type that the turbine blade 310 is inserted radially onto a cutout portion 301 which is formed at one position in the circumferential direction of the irregularities fabricated in the circumferential direction of the turbine rotor to embrace the rotor blade wheel 300 from the turbine blade side. Meanwhile, as shown in FIG. 15, the axial inserted blade root type is a type that the turbine blades 360 are inserted into the blade grooves of the rotor blade wheel 350 which are formed around the turbine rotor by forming plural blade grooves which are formed to have a recessed shape in the turbine rotor axial direction along the circumferential direction of the turbine rotor.
As shown in FIG. 16 and FIG. 17, when the above blade root types are compared, with respect to width a of the same rotor blade wheels 300, 350 the width of the turbine blades 310, 360 to be combined with them can be made larger for blade width b2 of the axial inserted blade root type than for blade width b1 of the saddle-shaped blade root type. It means that as far as both of them satisfy allowable stress, the turbine blade 310 of the axial inserted blade root type has higher load capability.
Taking the above advantage, the axial inserted blade root type is being used for various types of turbines which need a design to suppress a rotor span, namely a turbine rotor length, to a smaller level. In improvement design to improve the performance of an existing turbine, a change from the saddle-shaped blade root type to the axial inserted blade root type is also made in order to provide a small gap with an important function.
When a specified number of turbine blades are implanted in the rotor blade wheel, the last turbine blade is implanted as the stop blade. For the above-described conventional saddle-shaped blade root type, the stop blade is fixed to the rotor blade wheel by a fixing member 311 such as a fastening screw or a locking pin to prevent it from coming out in a radial direction as shown in FIG. 14.
Meanwhile, according to the above-described conventional axial inserted blade root type, a blade root portion 361 of the turbine blade 360 other than the stop blade which is the last turbine blade is inserted into the groove portion along the turbine rotor axial direction as shown in FIG. 15. And, the turbine blade 360 is inserted to reach a predetermined position, and a stop key 363 is inserted between a cutout portion 362 which is formed in the side surface on the circumferential direction side of the blade root portion 361 and two projections 351 which are formed on the top of the rotor blade wheel 350 between the groove portions. The insertion of the stop key 363 prevents the turbine blade 360 from moving in the turbine rotor axial direction. Thus, the turbine blades 360 other than the stop blade are implanted. Meanwhile, the stop key 363 cannot be inserted at the time of inserting the stop blade. Therefore, after a stop blade 360a is inserted into the groove portion, it is fixed with the adjacent turbine blades 360 by fastening screws 364 as shown in FIG. 18, and the stop blade 360a is prevented from moving in the axial direction.
As to the vibration design, if the turbine blade stands solely, there are large numbers of vibration modes which depend on the characteristics of the single blade. If vibratory force due to an external force such as steam power conforms to or comes close to such vibration modes, the resonant stress of the turbine blade becomes excessive, and if worst, it may result in breakage. Therefore, for example, JP-A 2000-18002 (KOKAI) designs to control the vibration modes according to a group blade structure which couples a plurality of the implanted turbine blades. According to JP-A 2004-52757 (KOKAI), it designs to control the vibration modes according to a perimeter group blade structure which couples all the turbine blades around the whole circumference.
As the perimeter group blade structure, there is a snubber structure that a shroud is provided at the tip end of a single blade, a working face is formed on the back side and ventral side of the shroud, and assembling is performed to contact one working face to the other working face of the adjacent turbine blade to configure the blades on the whole circumference as one group. In this snubber structure, vibration suppressing effect by friction of the snubber working face is high, and the number of vibration modes is limited because it is the perimeter group blade structure. Besides, since there is no a group head blade or a group tail blade different from the group blade structure, there is no significant point in the vibration modes, and a uniform vibration stress is produced in all the blades. Therefore, there are great benefits in view of the vibration suppressing design, such as easy control of vibration modes, and the snubber type blade is being used extensively.
If the adjacent snubbers are mutually contacted completely when the above snubber type blade implants the turbine blades into the groove portions, contact between the snubbers might become insufficient or a gap might be caused between the snubbers because the turbine blade body is floated upward or expanded by the centrifugal force, or differential thermal expansion or the like occurs between the turbine blades during the operation. If a gap is formed between the snubbers, vibration suppressing effect cannot be expected, and the turbine operation reliability is considerably deteriorated. Therefore, the conventional snubber type blade sometimes adopts a wedge-shaped snubber type that the standard rotor blade (ordinary blade) 370 and the offset rotor blade 380 from which expected is radial floating up by the centrifugal force are alternately arranged to aggressively contact the adjacent shroud portions 390 as shown in FIG. 19.
Control of a radial directional position of the snubber type blade to contact aggressively the shroud portion 390 is performed mainly by adjusting fitting gaps m, n, which are produced when blade root portions 370a, 380a are inserted into the groove portions formed in the rotor blade wheel 395 as shown in FIG. 20 and FIG. 21, to values different between the standard rotor blade 370 and the offset rotor blade 380.
Based on the two important design conditions of the strength design and vibration design described above, the latest turbines tend to adopt the axial inserted blade root type and the snubber type blade for the turbine blade of an important stage affecting the performance characteristics.
But, when the turbine blade is implanted by the turbine blade implanting method according to the above-described conventional axial inserted blade root type, it is relatively easy for the turbine blades other than the stop blade to conform the center of gravity of the blade root portion to the center of gravity line of the rotor blade by design calculation considering the stop key and its accompanying fixing structure portion. Meanwhile, for the stop blade, the rotor blade wheel and the axial end portion of the blade root portion are fixed by fastening screws, so that the effect against the centrifugal force is affected considerably.
When the above-described conventional wedge-shaped snubber type is adopted, it is necessary to accurately control the movements of the individual turbine blades in their radial directions, and designing, machining and assembling work must be performed with sufficient attention paid. Thus, it has a disadvantage that the production procedure becomes considerably complex. Especially, when operating, the centrifugal force applied to one turbine blade is also applied partly to the adjacent turbine blade via the shroud portion, so that the blade root portion of the adjacent turbine blade has a possibility of having a large stress, and both accurate design and exact assembling are required.
It has become apparent by accurate simulation calculation that when the above-described conventional snubber type blade is used, slight falling of the turbine blade caused at the time of assembling and increasing rotations, namely inclination, causes a nonuniform frictional force in the snubber, irregularities of the outer circumferential surface of the shroud, nonuniform stress or stress concentration caused in the hook portion of the blade root portion.